Measure a room for ventilation airflow
Your chosen ACH multiplied by room volume gives the target hourly airflow.
Back to calculator ↑Measure room volume
Measure internal length, width and height in metres. For sloping ceilings or irregular rooms, divide into sections and find the actual volume first.
Large built-ins or uneven ceilings can affect room volume. Enter dimensions in metres.
Choose an ACH deliberately
ACH is how many room volumes exchange per hour. The form’s 3 ACH is an example, not a requirement for every room type.
For a 56 m³ room, changing the chosen rate from 3 to 2 ACH changes the target from 168 to 112 m³/h. The comparison shows how ACH changes the flow target; choose the rate for the room’s use and any moisture or pollutant sources.
Worked default example
A 5 × 4 × 2.8 m room is 56 m³. At a chosen 3 ACH, flow is 168 m³/h or 2.8 m³/min.
Compare a catalog rating in the same units: 2.8 m³/min equals 168 m³/h. Use room volume, including height, rather than floor area alone.
Check installed airflow
Fan label airflow can differ from installed airflow. Duct bends, grilles and pressure reduce delivery; check fan performance data and installation details.
Long ducts, grilles and insufficient replacement air can reduce installed flow. Use the target when discussing fan placement and sizing with an installer.
If considering multiple fans, compare combined installed flow with the target, but placement and makeup-air paths still matter. Check stagnant areas, openings and noise as well as combined flow.
Common questions
Is ACH the fan size?
No. ACH is an exchange rate; airflow is expressed in m³/h.
Can I apply this directly to a bathroom or kitchen?
Check the requirements and use conditions for that space first.
Is a fan labeled 168 m³/h guaranteed sufficient?
No. Installed airflow may be lower than the label rating.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.